1. Field of the Invention
The present invention relates to image capturing apparatuses, image capturing methods, exposure control methods, and programs, and, more particularly, to an exposure control method for an image capturing apparatus that generates a combined image signal with a wide dynamic range by combining a long-exposure image signal and a short-exposure image signal.
2. Description of the Related Art
In image capturing apparatuses in the related art which use a solid-state pickup device such as a CCD (Charge Coupled Device), the quantity of light to be input into the pickup device (quantity of light exposure) is controlled by controlling an aperture value and an electronic shutter speed. That is, when a bright scene is captured, the quantity of light exposure is reduced so as to prevent the occurrence of overexposure that is caused by saturation of a signal output from an image pickup device. Conversely, when a dark scene is captured, the quantity of light exposure is increased so as to prevent the occurrence of underexposure.
However, when a scene with a strong contrast is desired to be captured (image capturing of a backlit subject or simultaneous image capturing of an indoor scene and an outdoor scene) using a solid-state image pickup device with an insufficient dynamic range, even if quantity of light exposure is controlled, overexposure may occur in a bright portion of the scene due to the saturation of the bright portion and underexposure may occur in a dark portion of the scene. As a result, both of these portions cannot be appropriately reproduced.
As a method of overcoming such a difficulty, U.S. Pat. No. 5,455,621 (corresponding to Japanese Unexamined Patent Application Publication No. 6-141229) discloses a method of separately obtaining bright area information and dark area information by using two different electronic shutter speeds in a field or changing an electronic shutter speed for each field, and combining the obtained pieces of information into a single image. This method is employed by apparatuses capable of capturing an image with a wide dynamic range (wide dynamic range cameras). In addition, U.S. Patent Publication No. 2006/033823 A1 (corresponding to Japanese Unexamined Patent Application Publication No. 2004-120205) and Japanese Unexamined Patent Application Publication No. 2002-84449 disclose apparatuses and methods for improving the quality of the above-described combined image.
In general, there are two exposure control modes, an exposure mode in which exposure control is performed in accordance with a user's setting and an automatic exposure (autoexposure) mode in which exposure control is automatically is performed. In the following description, the exposure mode in which exposure control is performed in accordance with a user's setting will be referred to as an “exposure setting mode” so as to be distinguished from the “automatic exposure mode.” In the exposure setting mode, a user optionally sets an exposure period for an image pickup device unit, an aperture value, and a value of a gain to be applied to a signal output from the image pickup device unit in accordance with the condition of a subject, the image of which is desired to be captured. In the automatic exposure mode, a control unit (microcomputer) included in an apparatus automatically sets the exposure period, the aperture value, and the gain value in accordance with a detection result of a luminance level of an image capturing area so that an appropriate luminance level can be obtained.
Here, if the exposure setting mode is selected in a camera that does not have a wide dynamic range image capturing function, a user controls the exposure period, the aperture value, and the gain value in accordance with the luminance level of a subject the image of which is desired to be captured (a main subject) so that an appropriate exposure level can be obtained. As a result, even if a luminance level of the background of the main subject is changed, the luminance level of the main subject can be maintained. On the other hand, however, overexposure may occur at a portion with a high luminance level on the background due to the saturation of the portion since exposure control is not performed in accordance with the luminance level of the background.
The above-described patent documents disclose a technique for improving image quality of a wide dynamic range camera by improving the method of combining images captured with two significantly different quantities of light exposure, but do not disclose a technique for improving the image quality of a combined image by performing image capturing with a wide dynamic range (combined image capturing) in the exposure setting mode.